CATV systems having one or more "premium" channels allocated for controlled access are well known in the prior art. Typically, such systems include a channel modulator and a scrambler for each premium channel. The channel modulator receives a program source, converts this information to IF and then to a CATV output channel frequency. To control access to the premium channel, the video and audio IF carriers from the modulator are intercepted and further modulated in the scrambler with a scramble signal. Normally, the scramble signal is a 15.75 Hz sine wave which amplitude modulates the IF video and audio carriers. This additional modulation suppresses the horizontal synchronization information in the video signal, thus preventing acquisition of the premium channel at the user location unless the user has an appropriate converter/decoder box. Decoding of the premium channel is effected by a decode circuit in the user's converter/decoder box, which removes the 15.75 Hz signal from the audio carrier, phase shifts this signal by 180.degree., and uses the phase shifted version thereof to cancel the 15.75 Hz signal on the video carrier.
The user of the CATV system described above can easily defeat the decode circuit in the converter/decoder box, thus allowing unauthorized and unlimited access to premium channels. Such access is also provided when the converter/decoder box has a defective decode circuit. The existence of unauthorized and defective boxes in the CATV system is costly to the system operator who receives a fee for each premium channel. One prior art approach to solving this problem is the use of a secured decoder filter in the converter/decoder box. In particular, a narrowband filter is assembled on a fragile substrate and spring-loaded in a housing. Any attempt to open the decoder box shatters the substrate, thus destroying the filter to prevent reception of the premium channel. This technique, shown in U.S. Pat. No. 4,097,894 to Tanner et al, is costly since it requires complete replacement of the decoder filter circuitry.
There is therefore a need to provide an effective method and apparatus for identifying unauthorized or defective converter/decoder boxes in a CATV system which is inexpensive to the system operator and obviates destruction of the decode circuitry in the user box.